Early in the third trimester may be best for Tdap vaccination during pregnancy

When immunizing pregnant women with Tdap vaccine, timing matters, according to research published online in JAMA October 9. In a study of 626 of women and their babies, researchers compared Tdap immunization during weeks 27 through 36 of pregnancy with no Tdap immunization and measured geometric mean concentrations (GMCs) of pertussis toxin antibodies in the cord blood of newborns. Among the women, 312 received Tdap vaccine, and 314 were unimmunized.

The researchers found that pertussis toxin antibodies were highest when the women received Tdap vaccine during weeks 27 through 30 of their pregnancies—early in the third trimester. The GMC of pertussis toxin antibodies at birth for newborns whose mothers received Tdap immunization was 47.3 IU/mL, compared with 12.9 IU/mL among newborns whose mothers were not immunized. GMC was highest when the women received Tdap vaccine at week 30, at 57.3 IU/mL. The amount of pertussis toxin antibodies in cord sera increased each week when the women received Tdap vaccine during weeks 27 through 29, peaked at week 30, and declined thereafter.

In their conclusion, the researchers noted that CDC recommends Tdap vaccination during weeks 27 through 36 of pregnancy, and that “optimal time to administer Tdap vaccine to maximize pertussis toxin antibodies at birth may be early in the third trimester, with the window of 27 through 30 weeks of gestation yielding the highest cord blood levels.”